1. Field of the Invention
This invention relates in general to electrical inductive apparatus, and more particularly, to magnetic core structure for electrical transformers.
2. Description of the Prior Art
Electrical transformers are well known in the art. Power transformers are those transformers used to transmit or distribute power in ratings larger than distribution transformers (usually over 500 KVA or 67 KV). Design of successful commercial transformers requires the selection of a simple form of structure so that the coils may be easy to wind and the magnetic circuit easy to build. At the same time, the mean length of the windings in the magnetic circuit must be as short as possible for a given cross-sectional area, to minimize resistance losses while the cross-sectional area of the magnetic core should be maximized for increased magnetic flux flow.
These apparently conflicting requirements have been partially resolved by practioners in the art by construction of transformer cores in which the core portion not encircled by windings includes extra laminations (thereby reducing magnetic flux density) while the portion encircled by windings remains of reduced size (thereby minimizing mean length of turn).
Two forms of transformers are in common use. When the magnetic circuit takes the form of a ring encircled by two or more groups of primary and secondary windings distributed around the periphery of the ring, the transformer is termed a core-form transformer. When the primary and secondary windings take the form of a common ring which is encircled by two or more rings of magnetic material distributed around its periphery, the transformer is termed a shell-form transformer. The characteristic features of a core-form transformer are a long mean length of magnetic circuit and a short mean length of windings and of a shell-form are short mean length of magnetic circuit and long mean length of windings.
It has been found that when additional core laminations are interleaved in a portion of a transformer core to increase cross-sectional area, that the voids created in the size transition region become greater in some parts of the core than others. This leads to a "sagging" of core laminations in the transition region and less than maximum core material density. In addition this "sagging" and non uniformity of core voids may tend to increase the noise made by the transformer in use.
Transformers, although they are classified as static apparatus, vibrate and radiate audible sound energy. There are two distinct and different sources of this sound energy. One source is auxiliary cooling equipment and the other source is the core of the transformer. Alternating flux flowing in the core laminations causes them to change length by the effect known as magnetostriction. While the magnetostriction affect is very small, being measured in parts per million, it is the principal cause of core noise. The effect is independent of direction of magnetization. Consequently, there are two core extensions per cycle of magnetization which accounts for the fact that the fundamental sound frequency is twice the excitation frequency. Since the magnetostriction characteristic is not a linear function of flux density, many harmonics are also generated. There are also magnetic forces set up between the laminations at the joint region where flux passes from one lamination to the other which also contributes to core noise.
Various means have been attempted to reduce noise produced in transformer cores. For example, U.S. Pat. No. 3,173,177-Franklin discloses one such noise-reducing arrangement for transformers. Franklin utilizes kinked laminations which tend to absorb the magnetrostriction.
There remains, therefore, a need in the art for an improvement in the structure of such laminated variable cross-section area core devices which will permit their use and realization of their advantages without the increased noise and less than optimum core density caused by void variations.